Independent Recombination by Nonalleles 



49 



AA' x AA' BB' x BB' AA' BB' x AA' BB' x V» 



Va AA 



Vi AA' 



figure 4-9. Genotypic re- 

 combination frequencies. 



y 4 a'a' 



show dominance in the hybrid condition), 

 each pair seeming to segregate independently 

 of all the others. Since the garden pea pos- 

 sesses a diploid number of seven pairs of 

 chromosomes, there are enough chromo- 

 some pairs for each pair of genes to be lo- 

 cated on a different pair of chromosomes. 



* Different Phenotypic Ratios - 



A monohybrid may show the phenotypic 

 effects of only one allele, some of the effects 

 of both alleles, or the complete effects of 

 both alleles. These phenotypic conse- 

 quences have already been designated as 

 complete, partial, and no dominance, re- 

 spectively. In the garden pea experiments 

 already discussed, complete dominance pro- 

 duced the 3 : 1 phenotypic ratio obtained 

 from a cross between identical monohybrids. 

 This necessitated breeding the offspring with 

 the dominant phenotype in order to identify 

 the 1:2:1 genotypic ratio predicted from 

 such crosses. Had no dominance or partial 



-See W. Bateson (1909). 



dominance occurred, the phenotypic and 

 genotypic ratios would have been identical. 

 Nevertheless, in all cases the recombining 

 genes retained their individuality, and the 

 specific ratios observed depended only upon 

 the dominance relation within the gene pair 

 — that is, the relation between the expres- 

 sion of one allele and that of its partner. 



Complete dominance also has no influence 

 upon the individuality or segregation of non- 

 allelic pairs of genes. Although the geno- 

 typic ratio expected from crossing two par- 

 ticular dihybrids has already been derived 

 (Figure 4-4), let us rederive this ratio, em- 

 ploying more general symbols for the genes, 

 using the branching-track method in a 

 slightly different way. Let A and A' be one 

 pair of alleles and B and B' another. Mating 

 AA' BB' by AA' BB' gives the genotypic 

 ratio seen in Figure 4-9. 



Notice here that among every 16 off- 

 spring, on the average, there would be 9 

 different genotypes in the ratio of 1:2:1:2: 

 4:2:1:2:1. How did this genotypic ratio 



